How is National Rail + Tube fare calculated?

A question about : How is National Rail + Tube fare calculated?

How is National Rail + Tube fare calculated?
For example if I start my journey with National Rail in Zone X and then arrive in Zone 1 and then catch the tube at that station and continue in the same Zone?

i.e.; Clapham Junction to London Victoria on National Rail, then London Victoria to Euston on the tube.

I can use a site to calculate the total... but how it's the total calculated?

Thanks!

Best answers:

• Am not sure how they calculate it but it is only calculated as one fare if you touch in again within a certain time.
  For example my son travelled from here to Victoria then tube to embankment. As he had a student card that didn't usually need a balance on it, I checked the single fare finder and put exact amount on his card. Yet he ended up in negative balance.
  When I called.tfl it was because he didn't go straight from the train to the tube on outward journey (the group was delayed by revenue inspectors as his stupid mates tried to get away with traveling on child tickets rather than students) it classed it as two trips and charged accordingly. Not sure what the time limit is though.
• All the fares are set out in a series of tables (available as PDF documents) - for National Rail fares, see this page:
  www.tfl.gov.uk/fares-and-payments/fares/national-rail
  Scroll down and click on Adult, 18+ Student, 11-15 etc to bring up the appropriate table.
  For journeys that include both National Rail and Tube, the fare you'll be charged is called a "through fare" and is included in those tables (in the PDFs the first set of fares is the "National Rail only" fare, the second set is the "through fare" - scroll down to see them).
  To take advantage of a through fare, you need to continue your journey within a certain amount of time. This is called making an Out of Station Interchange - more information here:
  www.oyster-rail.org.uk/interchanging-trains/
  The time limits for making an Out of Station Interchange are set out here:
  www.oyster-rail.org.uk/osi-list/
  For Victoria NR (National Rail) to Victoria LU (London Underground), the limit is 20 minutes.
  I think your son's stupid mates should pay him the difference!